gitlab-runner: Fix marshalling of TOML datetimes

As described in nixpkgs#356717, `gitlab-runner` now uses TOML date-time
values in its configuration (namely the `token_obtained_at` and
`token_expires_at` keys). As there is no JSON syntax for such values,
remarshal refuses to serialise them without the `--stringify` flag.
Thankfully, `gitlab-runner` seems to accept the stringified values
without any complaints, so we can side-step this issue readily.

modules/services/gitlab-runner.nix

@@ -133,7 +133,7 @@ let
       done
 
       # update global options
-      remarshal --if toml --of json ${configPath} \
+      remarshal --if toml --of json --stringify ${configPath} \
         | jq -cM ${escapeShellArg (concatStringsSep " | " [
             ".check_interval = ${toJSON cfg.checkInterval}"
             ".concurrent = ${toJSON cfg.concurrent}"